This invention relates to a pneumatic actuator which is suitable for any high speed and/or high force applications and more particularly although not exclusively, to a pneumatic actuator for a stores carriage and ejection system which is operable to carry stores such as an airborne weapon or missile, on an aircraft in flight and to eject those stores from the aircraft on demand.
U.S. Pat. No. 5,932,829 discloses a missile storage and ejector rack assembly which is to be used with a pneumatic release and ejection system of the type described in the U.S. Pat. Nos. 5,583,312 and 5,857,647. U.S. Pat. No. 5,904,323 discloses a missile store release system which can be used with any suitable pneumatic, hydraulic or pyrotechnic ejection mechanism. GB-A-2,078,912 discloses a missile launcher having fore and aft ejection rams and pyrotechnic ejection cartridges which are fired simultaneously to generate hot gas which is fed to the rams to extend them on ejection of the missile. Use of pneumatics results in a better facility for controlling the pressure that acts to extend the ejector rams than is the case with pyrotechnic cartridges. This leads to an ability to charge different rams of a multi-ram ejector system to different pressures which is advantageous.
An object of this invention is to provide a pneumatic actuator which is simple, compact and efficient and which when used in an airborne weapon carriage and ejection system has a performance which compares more favourably with pyrotechnic systems than do conventional pneumatic systems.
According to one aspect of this invention there is provided a pneumatic actuator for a stores carriage and ejection system which is operable to carry the stores on an aircraft in flight and to eject those stores from the aircraft on demand, the actuator having an actuating member which includes an actuating rod portion which extends from a differential area piston having two piston portions, the cross-sectional areas of which are different, the differential area piston being slidable in a closed chamber which is filled with gas under pressure when the actuator is operated and from which the actuating rod portion projects, one of the piston portions being slidably engageable in a cavity which is formed in an end wall of the closed chamber and which has a cross-section similar to that of said one piston portion, there being means operable when said one piston portion is engaged in said cavity to apply gas under pressure to a surface of said one piston portion which is formed at an end thereof remote from the other piston portion whereby to apply an actuating thrust to the actuating member and to displace said piston portion from the cavity so that the greater cross-sectional area of the differential area piston is subjected to the gas under pressure within the closed chamber.
In a preferred embodiment releasable hold back means operable to hold the piston in the location that is furthest from the location of the piston at which the volume within the chamber is at its maximum are provided.
Hence initial movement of the piston (once the piston has been released when any releasable retaining means or said releasable hold back means are provided) will be effected under a low force until said one piston portion has been urged out of said cavity, such displacement of the piston allowing the largest possible force to be exerted on the piston by the action of compressed gas and the transfer of the greater possible portion of the expansion energy of compressed gas in the chamber to any load connected to the piston.
In a preferred embodiment the chamber is a cylinder which is closed at one end and is separated by a wall from another chamber which is charged with compressed gas in use of the actuator, the wall forms an aperture which serves as said cavity, the two piston portions form a stepped piston, and the actuating rod portion is a piston rod, the stepped piston being mounted at one end of the piston rod by the one of the two piston portions that has the smaller cross-sectional area, wherein the piston rod extends through the aperture and the other chamber and has a cross-sectional area which is smaller than that of the aperture which is sized to slidably receive said one piston portion of the stepped piston, the releasable hold back means being operable to hold the piston adjacent to the wall with said one piston portion received in the aperture.
Preferably the dimensions of the piston rod and the aperture allow substantially unrestricted gas flow between the other chamber and the cylinder when said one portion of the piston is displaced from the aperture towards the closed end of the cylinder. Valve means may be provided which are operable to place both sides of the piston in communication one with the other for a return stroke of the piston when the piston has been driven to the closed end of the cylinder by the action of compressed gas in the other chamber. The valve means may comprise a bi-stable shuttle valve which is switched from one of its states to the other by arrival of the piston at either end of the cylinder.
The releasable hold back means conveniently include an external latch. The latch may include a pivoted arm having an arcuate end portion which functions in the manner of a zero lift cam, the arm being adapted to be positioned so that the end portion abuts an abutment carried by a member which is coupled to the piston whereby to hold the piston in said location, the piston being released by displacement of the arm so that the end portion is moved to one side of the abutment, whereby the abutment is free for movement with the piston. Means operable to displace the arm may be provided, those means together with the arm being dynamically balanced. The external latch may include a collapsible pair of under-centre strut links which are coupled with the piston, which are held by the latch in stable equilibrium against the action of compressed gas on the piston and which are arranged to collapse and allow movement of the piston when the latch is released.
According to another aspect of this invention, there is provided a stores carriage and ejection system which is operable to carry stores on an aircraft in flight and to eject those stores from the aircraft in flight on demand, said system including at least one fluid pressure operable ejection ram which includes a piston which is slidably mounted in a cylinder, and compressed gas storage means adapted to supply compressed gas as the working fluid for operating said at least one ejection ram to eject the stores from the aircraft, wherein the interior of the cylinder of the or each said at least one ejection ram serves as said compressed gas storage means.
In such a stores carriage and ejection system which includes at least two fluid pressure operable ejection rams, the cylinders of the ejection rams may be charged to different pressures for ejecting the stores as may be required when the center of gravity of the stores is not equally spaced from each ram piston.
Preferably the piston of said at least one ejection ram is a differential area piston having two piston portions, the cross-sectional areas of which are different, and an actuating rod portion which projects substantially co-axially with the two piston portions out of the cylinder which is a closed chamber which is filled with gas under pressure when the ejection ram is operated, the one of the piston portions that has the larger cross-sectional area being slidably engagable in a cavity which is formed in an end wall of the closed chamber and which has a cross-section similar to that of said one piston portion so that the piston is urged towards said end wall by the gas pressure loading thereon when said one piston portion is engaged in said cavity and said closed chamber is filled with gas under pressure, said end wall being remote from a location in the closed chamber from which the actuating rod portion projects out of the closed chamber, there being means operable to apply gas under pressure to a surface of said one piston portion when said one piston portion is engaged in said cavity, said surface being formed at an end of said one piston portion which is remote from the other piston portion whereby to apply an actuating thrust to the actuating rod portion and to displace said one piston portion from the cavity so that the greater cross-sectional area of the differential area piston is subjected to the gas under pressure within the closed chamber.
Throttling means may be provided to restrict flow of gas under pressure that is applied to said surface of said one piston portion. Additionally or alternatively said surface of said one piston portion may be formed with a recess into which the flow of gas under pressure that is applied to said surface is introduced. Furthermore, the end of said one piston portion that forms said surface may be tapered.
The piston may be provided with a perforated flange on said other piston portion, the flange being perforated so as to allow free flow of gas therethrough and being adapted to locate the piston within the closed chamber as the ejection ram is operated to eject the stores. Cushioning means may be provided for cushioning arrest of the piston at the end of its stroke to extend the ejection ram. The piston may be retracted manually from the end of its stroke to re-engage said one piston portion in said cavity after extension of the ejection ram. Alternatively resilient means may be provided to effect automatic retraction of the piston. Conveniently the piston is tubular and said cushioning means and/or said resilient means may comprise a plunger which is fixed with respect to the cylinder and which is slidably engaged within the interior of the hollow piston, there being resilient means which may be a spring arrangement or compressed gas which act between the plunger and the piston to urge the piston to said end wall.
According to a further aspect of this invention there is provided a stores carriage and ejection system which is operable to carry stores in an aircraft in flight and to eject those stores from the aircraft in flight on demand, said system including at least one fluid pressure operable ejection ram which includes a piston which is slidably mounted in a cylinder, and compressed gas storage means adapted to supply compressed gas as the working fluid for operating said at least one ejection ram to eject stores from the aircraft, wherein said compressed gas storage means includes a gas pressure intensification system which is operable to intensify the pressure of gas supplied to operate said at least one fluid pressure operable ejection ram.
Preferably said stores carriage and ejection means includes pneumatically operable latch means which are provided for retaining stores which are to be carried for ejection and which are actuated to release those stores for ejection, said gas pressure intensification system also being operable to intensify the pressure of gas supplied to operate said latch means.
Conveniently said gas pressure intensification system includes a gas pressure intensifier having an inlet for receiving compressed gas stored in said compressed gas storage means and an outlet for connection to the or each fluid pressure operable device of the stores carriage and ejection system, and a valve which is operable to close said outlet and thereby to retain gas within the gas pressure intensifier until a certain rapid pressure rise is detected whereupon it opens fully to allow the intensified gas pressure to be applied to the or each fluid pressure operable device.
The gas pressure intensifier may comprise a stepped piston slidable in a closed stepped cylindrical chamber, the stepped piston being hollow and having two ends, the larger diameter end being open and the smaller diameter end being closed, the stepped piston dividing the interior of the stepped cylinder into an annular space which is formed around the smaller diameter portion of the stepped piston between the steps in the piston and the cylinder, an end space formed in the smaller diameter portion of the cylinder between the stepped piston and the closed end of the smaller diameter portion of the cylinder and a third space which comprises the interior of the stepped piston and the remainder of the larger diameter portion of the stepped cylinder between the stepped piston and the closed end of the larger diameter portion of the stepped cylinder, the inlet of the gas pressure intensifier communicating with the annular space and the outlet communicating with the end space, there being one way valves in the stepped portion of the stepped piston and in the closed end of the smaller diameter portion of the stepped piston, said one-way valves allowing flow from the annular space to the third space when a certain pressure has been built up in the annular space and allowing flow from the third space to the end space when another certain pressure has been built up in the third space.
The valve that closes the outlet of the gas pressure intensifier conveniently comprises a differential area piston valve having an obturating portion at a smaller diameter end of the piston valve and a larger diameter piston portion which slides in a bore in a housing of the valve, and a passage of flow restricted dimensions which places the outlet of the gas pressure intensifier in restricted communication with a space within the housing which is bounded by the larger diameter piston portion, there being a coil spring urging the piston valve to seat the obturating member to close the outlet and a plenum chamber with an outlet formed between the larger diameter piston portion and the obturating member, the outlet of the gas pressure intensifier being in communication with the outlet of the plenum chamber when the piston valve is unseated.